Positron emission tomography (PET) has become increasingly widely established in the medical diagnostics field in the last several years in addition to magnetic resonance tomography (MR). Whereas MR is an imaging method for representing structures and slice images in the interior of the body, PET enables a visualization and quantification of metabolic activities in vivo.
PET exploits the special properties of positron emitters and positron annihilation for quantitatively determining the functioning of organs or cell regions. For this purpose suitable radiopharmaceuticals labeled with radionuclides are administered to the patient prior to the examination. Upon decaying, the radionuclides emit positrons which after a short distance interact with an electron, thereby resulting in the occurrence of a so-called annihilation. During this process two gamma quanta are generated which fly apart in opposite directions (offset by 180°). The gamma quanta are detected within a specific time window by two oppositely disposed PET detector modules (coincidence measurement), as a result of which the site of the annihilation is pinpointed to a position on the connecting line between the two PET detector modules.
For detection purposes the PET detector module must generally cover a major part of the length of the gantry. The module is subdivided into detector elements having a side length of a few millimeters. Upon detecting a gamma quantum each PET detector element generates an event record which indicates the time and the detection site, that is to say identifies the corresponding detector element. This information is transferred to a fast logic circuit and compared. If two events coincide within a maximum specified time interval a gamma decay process is assumed to have occurred on the connecting line between the two associated PET detector elements. The PET image is reconstructed by way of a tomography algorithm, referred to as back-projection.
It is advantageous to allow MR and PET examinations simultaneously within one device. In this case both morphological MR data and PET data can be acquired within one measurement pass. For this purpose it is necessary to arrange the PET detectors inside the MR device such that the imaging volumes ideally coincide. For example, the PET detectors can be arranged on a supporting structure (carrier tube, gantry) located inside the MR device. These can be, for example, 60 detectors in an annular arrangement on the carrier tube. A cooling terminal and electric supply leads are required for each of the detectors, which can also be combined into detector blocks. These also have to be arranged in the MR device. In addition a number of signal processing units are required which are likewise arranged in the MR device. These are connected to the detectors by way of the electric supply leads and are used for signal processing.
Frequent maintenance interventions are necessary on the PET detectors as well as on the signal processing units. In this case the affected units and detectors may need to be replaced or must be removable and accessible for repair purposes. In maintenance activities of the kind any impact on the remaining device, particularly on the unaffected PET detectors and signal processing units, should be avoided as far as is practically possible.